Hey William!
I know there are plugins that allow you to do that. I think there is something called Soundflower that allow you to manipulate the sound in and sound out of the audio in Mac. Basically, I had a whole load of those plugins fitted in as I wanted to record Skype audio through Garageband for a Uni project (and was broke to purchase a paid-for app). While I am not a pro in telling you how to configure it, I can say that there exist a whole ton of plugins that allow you to do so! Here is a link...you may have to mine out more info to tweak it. Soundflower.
Cheers and all the best.
Zen!

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